As the integration density of semiconductor devices increases, the employment of dielectric layers having relatively high dielectric constants is desirable for potentially securing high capacitance in relatively small areas. One representative example of a dielectric layer having a relatively high dielectric constant is a tantalum oxide (Ta2O5) layer.
Notwithstanding its relatively high dielectric constant, the use of the tantalum oxide layer is potentially problematic in that it typically is potentially highly reactive with polysilicon present as a lower electrode in a conventional capacitor. In particular, polysilicon is usually oxidized in the formation of a tantalum oxide layer or a thermal treatment process after the formation of a tantalum oxide layer.
In an attempt to address this problem, lower electrodes have been formed from materials which are believed to be relatively difficult to oxidize. Examples of such materials include noble metals such as platinum (Pt), ruthenium (Ru), or iridium (Ir), or a conductive metal nitride layer such as titanium nitride (TiN). However, the use of a noble metal or metal nitride also presents potential problems. A conventional tantalum oxide layer is typically formed by chemical vapor deposition in an oxygen atmosphere using pentaethoxide tantalum (PET), Ta(OCH3)5, or TaCl5 as a tantalum source. Oxygen (O2), water (H2O), hydrogen peroxide (H2O2), or nitrous oxide (N2O) is employed as an oxygen source in such a method. Notwithstanding any advantages associated therewith, a composition of these source gases often negatively impacts the coverage of the tantalum oxide layer, presumably due to the oxidation of the lower electrode. For example, when a ruthenium (Ru) layer is used as a lower electrode, the surface of the Ru layer is oxidized by an oxygen source which results in the formation of RuO2.
The formation of RuO2 is carried out so as to minimize or prevent the formation of a tantalum oxide layer. The formation of a tantalum oxide layer often occurs when such a layer is used as a dielectric layer in a cylindrical or concave-shaped capacitor having a large aspect ratio. In such an instance, the tantalum oxide layer is not deposited on the lower portion of the cylindrical opening in the Ru electrode. Instead, the tantalum oxide layer is typically deposited on the upper portion to deteriorate step coverage of this layer.
There is a need in the art to address the above-mentioned problems in the art. More specifically, there is a need in the art to provide a semiconductor capacitor having a tantalum oxide layer present thereon with a more uniform thickness. It would be desirable to obtain a capacitor having a relatively high aspect ratio.